Being a Prisoner of War would not rule out passing a captive over to a properly constituted international tribunal. What would rule that out would be, if the US Government did not wish this to be done. Which appears to be the case.

The main charges which such a tribunal would be dealing with would be those involving the use of chemical weapons in the aggression against Iran, and indeed, that whole war. Even if the current US Government was not apparently uncomfortable with the whole notion of international tribunals, that is not an aspect of Saddam's behaviour which it would want given too much attention.